Chemical refining and processing methods frequently involve converting a liquid to a vapor with a reboiler. One such example for a reboiler is a tube-in-shell steam reboiler. In the steam reboiler, steam is condensed by transferring thermal energy to a process liquid. The process liquid is vaporized by the thermal energy and returned to the process as process vapor, where, for example in a fractionation column, the vapor will interact with other molecules and separate into different components within the fractionation column.
Such a reboiler often utilizes one or more control valves to adjust the pressure or temperature of the various streams associated with the reboiler. For example, a control valve is utilized to reduce a pressure of the steam stream to limit the ultimate temperature of the steam in a heat exchanger within the reboiler. Additionally, a control valve is utilized with boiler feedwater mixed with the steam to remove any superheat. The boiler feedwater is at a much higher pressure and therefore, a control valve is needed to reduce the pressure of the boiler water, allowing it to be mixed with the steam. Furthermore, once the steam is condensed in the heat exchanger within the reboiler, a control valve is utilized with the hot condensate to modulate the duty of the reboiler.
While the reboilers achieve their intended purposes, the control valves are a source of energy loss associated with the pressure reduction of the streams. Specifically, in the control valve, mechanical energy is dissipated by the valve in a thermodynamically adiabatic, highly irreversible process. Since the energy is removed in such an irreversible process, via the pressure reduction, without recovery, the energy is lost.
Because the pressure reduction across the control valve is irreversible, it results in a lower pressure steam with greater amounts of superheat than at the inlet of the valve. For the purposes of maximizing the utility of the heat exchanger surface area, the steam should condense immediately upon entering the heat exchanger to minimize the heat exchanger surface area. With high amounts of superheat, for example greater than 20 degrees C., a large amount of exchanger surface area is involved in slow heat transfer of sensible heat removal from the steam. To ensure quick condensation of the steam in the heat exchanger, some water is added to the steam downstream of the control valve to minimize the steam superheat. This addition of water adds cost to the system and therefore it is desirable to be minimized or eliminated.
Returning to the energy dissipated across the control valve; this dissipated energy is often associated with energy added to the system. Thus, there is an inherent inefficiency in the process associated with supplying energy only to remove it without recovery. In the past, the cost of recovering this energy has not been justified; however, with increased mandates for improvements in energy efficiency and reduction of greenhouse gas emissions, the elimination of these oft-overlooked inefficiencies provides a means to address these new mandates. Moreover, this inefficiency is an opportunity for processors to lower operating costs and, thus increase profits.
Therefore, there is a need for an effective and efficient device and process for recovering energy from the reboiler. Additionally, it would be desirable for such devices and processes to allow processors to analyze the inefficiency of removing energy that has been added and adjust processing conditions to minimize the added energy without impacting the overall throughput of the processing/refining unit.